1. Field of the Invention
The present invention relates to a substrate processing method and a substrate processing apparatus, and in particular, to a substrate processing method for a substrate processing apparatus having an upper electrode plate made of silicon.
2. Description of the Related Art
Conventionally, a substrate processing apparatus that subjects a semiconductor wafer (hereinafter referred to merely as a “wafer”) as a substrate to plasma processing has a chamber in which a wafer is housed and of which interior is evacuated, and a susceptor that is disposed in a lower part of the chamber and on which the wafer is mounted, and a showerhead that is disposed in an upper part of the chamber and supplies a process gas into the chamber. The showerhead has an upper electrode plate that is disposed in a manner opposed to and parallel to the susceptor.
In the substrate processing apparatus, a radio-frequency power source is connected to the susceptor, and the susceptor thus acts as a lower electrode unit. Also, another radio-frequency power source is connected to the upper electrode plate, and the showerhead thus acts an upper electrode unit. The susceptor and the showerhead produce an electric field in the chamber, whereby the process gas supplied into the chamber is turned into plasma.
In the case where radio-frequency electrical power is applied to the upper electrode plate, if the frequency of the radio-frequency electrical power increases, the effects of inductance of the upper electrode plate on the electric field produced by the upper electrode plate cannot be ignored. As a result, the intensity of the electric field produced from a central portion of the upper electrode plate increases, and the density of plasma in a part facing the central portion of the upper electrode plate increases. That is, there have been problems that, for example, the distribution of plasma in the chamber becomes uneven, and the distribution of etch rates in wafers becomes uneven.
To cope with this, the applicant of the present invention has proposed that a cavity portion is provided in a surface of the central portion of the upper electrode plate opposite to a surface that faces plasma (hereinafter referred to as the “plasma surface”), and resonance is produced in the cavity portion to produce an electric field orthogonal to the upper electrode plate (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2001-298015 (FIG. 3)). This makes it possible to bind the electric field in the cavity portion and the electric field in the upper electrode plate together and control the electric field in the central portion of the upper electrode plate, thus eliminating the unevenness in the distribution of plasma in the chamber.
Moreover, in recent years, a higher level of micromachining has been required for etching processing as plasma processing. To realize micromachining, high-density plasma has to be produced, and to achieve this, the frequency of radio-frequency electrical power has to be increased, and/or the electrical energy of radio-frequency electrical power has to be increased so as to keep plasma in a satisfactory dissociated state.
However, although the density of plasma can be increased by increasing the frequency and amount of radio-frequency electrical power, the temperature of the upper electrode plate increases at the same time because ion fluxes irradiated on the upper electrode unit (showerhead) increase.
The showerhead of the conventional substrate processing apparatus has a cooling plate that contacts and cools the upper electrode plate, but the above described cavity portion exists between the central portion of the upper electrode plate and the cooling plate. It is thus difficult to efficiently cool the central portion of the upper electrode plate, and the central portion of the upper electrode plate may be heated to a high temperature only by cooling it without an aim.
On the other hand, the upper electrode plate is made of p-type silicon doped with boron or the like (the initial specific resistance value is about 75Ω·cm), and it is known that the specific resistance value of the p-type silicon changes if it is maintained at a high temperature for a long period of time. In particular, if the central portion of the upper electrode plate made of the p-type silicon is maintained at a high temperature for a long period of time, oxygen mixed as impurities in the p-type silicon acts as donors to supply free electrons to eliminate holes produced by the boron. Thus, if the specific resistance value of the central portion of the upper electrode plate increases and then, the holes are completely eliminated (if the p-type silicon turns into n-type silicon), the specific resistance value decreases. If the specific resistance value of the central portion of the upper electrode plate decreases, the intensity of the electric field produced from the central portion of the upper electrode plate increases to cancel the effect of providing the above-mentioned cavity portion. As a result, the density of plasma in an area facing the central portion of the upper electrode plate increases.
Therefore, only by cooling the central portion of the upper electrode plate without an aim, the distribution of plasma in the chamber cannot be stabilized, and the unevenness in the distribution of plasma cannot be eliminated.